The use of intra-operative probes to assist surgeons in defining the boundaries of cancerous lesion during surgical procedures performed following the injection of the patient with a radiopharmaceutical such as a positron labeled fluorodeoxyglucose (FDG) or similar materials is becoming more and more commonplace. The problem of detecting the relatively weak beta signal from tissue in the vicinity of a probe inserted into the proximity of the lesion from the relatively strong gamma background field emitted from other regions of the body remains, however, perplexing. Furthermore, it has been discovered that positron emitting radiopharmaceuticals such as FDG create two high energy gamma rays or photons when the positron collides with an electron. The presence of these highly penetrating gamma rays greatly reduces the observed lesion-to-background contrast hoped to be gained by the use of these radiopharmaceuticals when conventional prior art techniques and systems are used in an attempt to detect the beta particles. It would therefore be highly desirable to have a small probe useful for intra-operative procedures that is highly sensitive to beta emissions while being insensitive or relatively so to gamma radiation.
As a consequence, a number of solutions have been proposed to solve this problem. For example, U.S. Pat. No. 5,744,805 to Raylman et.al. proposes the use of a probe system that utilzes an ion-implanted silicon charged-particle detector for generating an electrical signal in response to received beta particles.
U.S. Pat. No. 5,008,546 to Mazziotta et. al. discloses a probe comprised of two plastic scintillators, in a photocathode, optically coupled to corresponding light pipes. One of the plastic scintillators is shielded against beta radiation while the other is left to detect both beta and gamma radiation. The gamma radiation sensitivity of the two probes is empirically established and used as a weighted factor to subtract the outputs of the two probes to leave a signal indicative of the beta radiation emitted by the radiolabeled tissue.
U.S. Pat. No. 5,753,917 to Engdahl, although not directed specifically at the design of an intra-operative probe, describes a so-called phoswich or scintillation crystal assembly having multiple crystal layers for interacting with various photon or gamma ray energy levels so as to distinguish therebetween.
The interoperative probe of the present invention utilizes a significantly different approach to the problem of gamma ray interference with beta particle measurement that relies on a beta particle probe that is blind to gamma radiation, and hence, uninfluenced thereby in the detection process.
Secondary electron amplifiers have long been used in combination with the aforesaid scintillators to enhance the output of scintillator materials, or photocathodes, excited by gamma or other similar high energy emissions. Similarly, secondary electron multipliers have been used alone for the detection of beta particles. Some such devices are commonly referred to as photomultiplier tubes (PMT) or microchannel plates (MCP). Fundamentally, these devices comprise one or a series of dynodes which, when impacted by a beta particle, generally an electron, produce a shower of electrons thereby amplifying the number of particles available for detection. A variety of such devices have been designed, including flat plates, and "Chevron" devices that comprise layers of angularly displaced dynodes. Such devices are well known in the art and have broad usage in a number of electron and beta sensing and amplification devices.
The use of such devices for the detection of beta particles, however, has always required placement of the beta emitting sample under evaluation or the beta producer, in the case of a scintillator, into the vacuum with the PMT or MCP device to assure minimization of interference from air or gas between the source of the beta particle and the amplifier.